The present application relates generally to the field of datalink communications. More particularly, the application relates to a system and method to detect the presence of a digitally modulated waveform on a transmission channel.
Very High Frequency Digital Link (VDL) Mode 2 is a type of datalink communication system used in Aeronautical communication systems. VDL Mode 2 is used to improve air traffic management, leading to improved operational efficiency among other improvements, in the aircraft industry.
VDL Mode 2 is a carrier sense multiple access (CSMA) packet data system. CSMA is an access technique that requires a user check the transport medium using a listen function prior to transmitting. If the transport medium is busy, the user is required to hold for a period of time prior to reinitiating a transmitting function. If two stations transmit simultaneously on the transport medium, the resultant collision can corrupt both messages. Further, the physical layer of VDL Mode 2 further utilizes a 31.5 kilobits per second data rate Differential Phase Shift Keying (D8PSK) digital waveform using raised cosine filtering with 0.6 excess bandwidth.
Accordingly, VDL Mode 2 allows more available bandwidth by providing a faster and more reliable transmission speed while also enhancing communications with flight controllers and pilots.
However, to implement the CSMA access technique, it is necessary to prevent unintentional transmissions by determining channel activity. To prevent unintentional transmissions, the energy level of the channel is checked in decibels (dBm) or microvolts. An energy level threshold, used to prevent the collision of messages, has been set by the Radio Technical Commission for Aeronautics (RTCA). If the channel is above a certain energy level threshold, currently approximately −87 dBm, the channel is considered busy and the user must delay transmitting until the channel is clear. If the channel energy level is below the threshold, the user is free to transmit.
Traditionally, the energy level of the channel has been monitored by examining the automatic gain control (AGC) level of a receiver monitoring the channel. The AGC level is determined by monitoring the automatic gain control response on a receiver. However, the AGC level is responsive to any kind of energy on the channel, whether the source is a VDL Mode 2 signal, an unmodulated carrier, or other interference. Accordingly, monitoring the AGC level can create many false positives when detecting the presence of a VDL Mode 2 signal. False positives can degrade network efficiency and cause system indications to an operator of no communications.
Therefore, there is a need for a system and method for monitoring a communication channel and to distinguish a desired type of modulated waveform from ambient interference sources and to prevent false positives. There is also a need for a system and method for monitoring a communication channel carrying VDL Mode 2 signals to detect the presence of VDL Mode 2 signals and distinguish the signals from ambient interference sources to prevent false positives.